1. Field of the Invention
The present invention relates to a device for adjusting an angular phase difference and in particular to a device for adjusting an angular phase difference between an engine crank shaft and an engine cam shaft.
2. Description of the Prior Art
A conventional device of this kind is disclosed in U.S. Pat. No. 2,861,557, for example. In the conventional device, a drive member to be driven by an engine crank shaft is rotatably mounted on an engine cam shaft having a cam member which acts on a tappet. A hub is also fixedly mounted on the cam shaft so as to be enclosed by the drive member. The drive member is provided with a pair of equi-spaced vanes, each of which is extended inwardly in the radial direction so as to be brought into sliding engagement with an outer surface of the hub. The hub is also provided with a pair of equi-spaced vanes, each of which is extended outwardly in the radial direction so as to be brought into engagement with an inner surface of the drive member. Thus, between each of the vanes of the drive member and each of the vanes of the hub there is defined an oil chamber to which fluid under pressure is supplied. In such a construction, when fluid under pressure is supplied into each oil chamber, the volume thereof is increased, thereby increasing an angular phase difference between the drive member and the hub. Thus, the cam shaft is advanced through an angle relative to the crank shaft.
However, since fluid in each chamber serves for transmitting the rotational torque from the crank shaft to the cam shaft in the above-mentioned construction, the pressure of the fluid has to overcome the rotational torque from the crank shaft in order to retain or hold the advanced condition of the cam shaft relative to the crank shaft. This means that a high powered oil pump as to be used. In other words, fluid which already circulates in an engine system for the lubrication thereof cannot be utilized.
Further, in the conventional device, before advance of the cam shaft relative to the drive member, a ball holding the connection therebetween is moved outwardly in the radial direction as a result of centrifugal effect on the ball. This means that the cam shaft may not be advanced relative to the drive member while the engine is operated at a low speed.